A conventional PWM motor driving apparatus disclosed in Japanese Patent Application Non-examined Publication No. H05-38184 is known as realizing low power consumption of a brush-less motor (hereinafter simply referred to as a motor). FIG. 12 shows a structure of a conventional motor driving apparatus. In FIG. 12, position detecting element 1500 uses, e.g., Hall elements. Each one of those position detecting elements 1500 detects a position of respective rotor magnets (not shown) of the motor, and outputs a detection signal to conductive state instructing-circuit 1300 via position detecting circuit 1400. Instructing circuit 1300 outputs timing signals UHo, VHo, WHo, UL, VL, and WL in order to switch driving coils 1, 2 and 3.
Phase comparator 1204 compares a phase of frequency signal Fsp proportionate to a motor speed with a phase of reference frequency signal Fref. Phase comparator 1204 outputs phase-error signal PD, which indicates the phase difference between the foregoing two signals, to phase error amplifier 1202. Amplifier 1202 amplifies signal PD and outputs signal Vth to a non-inverting input terminal of PWM comparator 1201. An inverting input terminal of PWM comparator 1201 receives signal Vosc supplied from oscillator 1203. PWM comparator 1201 compares signal Vth with signal Vosc, and outputs signal Vd. AND gate 1101 receives signal Vd and the foregoing signal UHo, and outputs signal UH to the base of transistor 1009. AND gate 1102 receives signal Vd and signal VHo, and outputs signal VH to the base of transistor 1007. AND gate 1103 receives signal Vd and signal WHo, and outputs signal WH to the base of transistor 1005.
Signal UL is fed into the base of transistor 1021, signal VL is fed into the base of transistor 1020, and signal WL is fed into the base of transistor 1019.
The collector of transistor 1008, which forms a Darlington pair with transistor 1009, is connected to the collector of transistor 1021, and this junction point is connected to a first terminal of driving coil 1 of the motor. The collector of transistor 1006, which forms a Darlington pair with transistor 1007, is connected to the collector of transistor 1020, and this junction point is connected to a first terminal of driving coil 2 of the motor. The collector of transistor 1004, which forms a Darlington pair with transistor 1005, is connected to the collector of transistor 1019, and this junction point is connected to a first terminal of driving coil 3 of the motor. Between the emitter and the collector of transistor 1008, diode 1003 is connected. Between the emitter and the collector of transistor 1006, diode 1002 is connected. Between the emitter and the collector of transistor 1004, diode 1001 is connected. Between the collector and the emitter of transistor 1021, diode 1018 is connected. Between the collector and the emitter of transistor 1020, diode 1017 is connected. Between the collector and the emitter of transistor 1019, diode 1016 is connected. Respective second terminals of coils 1, 2 and 3 are connected with each other.
As discussed above, transistors 1009, 1008 and diode 1003, transistors 1007, 1006 and diode 1002, transistor 1005, 1004 and diode 1001 form an upper arm. Transistor 1021 and diode 1018, transistor 1020 and diode 1017, transistor 1019 and diode 1016 form a lower arm. Coils 1, 2 and 3 are connected between the upper arm and the lower arm.
The respective signals UHo, VHo, WHo, UL, VL and WL change their High and Low states responsive to positions of the rotor magnets, and conductive states of coils 1, 2 and 3 change sequentially, so that the motor rotates.
FIG. 13 illustrates an operation of the conventional driving apparatus shown in FIG. 12, and particularly shows the operation of obtaining signal Vd supplied from PWM comparator 1201. Phase comparator 1204 is formed of, e.g., flip-flops. As shown in FIG. 12, phase comparator 1204 uses respective rising edges of signal Fref and signal Fsp to set or reset comparator 1204 (flip-flops), and outputs signal PD. Phase-error amplifier 1202 provides signal PD with integrating amplification, and amplifies signal PD to a voltage level responsive to the duties of High/Low of signal PD, and outputs signal Vth. PWM comparator 1201 compares signal Vosc that is a carrier frequency signal of PWM driving with signal Vth, and outputs signal Vd having High/Low duties responsive to a voltage of signal Vth.
As shown in FIG. 13, when a phase difference between signal Fref and signal Fsp is large, a pulse width of signal PD becomes large (a period of High becomes longer), so that a voltage of signal Vth rises. As a result, a period of High of signal Vd increases, and an ON period of PWM driving increases, so that the motor is accelerated. On the contrary, when the phase difference between signal Fref and signal Fsp is small, a pulse width of signal PD becomes narrow (a period of High becomes shorter), and a voltage of signal Vth lowers. As a result, a period of Low of signal Vd increases, and an OFF period of PWM driving increases, so that the motor is decelerated. Signal Vd supplied from PWM comparator 1201 is logically synthesized with respective signals UHo, Vho and WHo supplied from conductive state instructing circuit 1300, so that the first conduction switch signals UH, VH and WH are obtained. In other words, the ON/OFF signal, i.e., high-level/low-level signal, having a duty proportionate to an output level from phase-error amplifier 1202 allows the group of transistors in the upper arm to perform chopping operations. As discussed above, the conventional driving apparatus shown in FIG. 12 makes the group of transistors in the upper arm perform the chopping operation, thereby controlling the rotation of the motor with a low power consumption.
In the foregoing conventional driving apparatus, when respective transistors in the upper arm are in the PWM operation, even during the OFF period of the PWM operation, the energy stored in respective coils is consumed as a regenerative current in flywheel diodes coupled in parallel with the respective transistors in the lower arm, i.e., the counterpart of the transistors in the upper arm. As a result, a lot of power is lost in the flywheel diodes. Meanwhile, a driving method, in which either one of the transistor-groups in the upper arm or the lower arm performs PWM chopping drive, is referred to as a single side PWM driving method.
A synchronous rectification PWM driving method is disclosed in Japanese Patent Application Non-examined Publication No. H05-211780. This method is proposed to overcome the foregoing problem, and turns on the transistor counterpart of a transistor which is turned off, namely, during an OFF period of PWM. This mechanism allows the energy stored in the driving coils to regenerate via the driving transistor having a smaller ON resistance than that of the flywheel diode, thereby reducing the power loss.
In general, a motor used in information devices requires a large amount of current in a start-up period for being ready to work in minimal time. This is a critical point to evaluate the performance of the motor. Therefore, several times of starting current is fed to the driving coils of a motor more than that of the regular rotating condition.
However, in the structure discussed above, a higher PWM frequency set for satisfying a rotational accuracy required to a motor also raises the PWM frequency to the higher one in the start-up period which does not require such a severe rotational accuracy. As a result, power loss (switching loss) generated at ON/OFF of the driving transistor increases, which entails heat generation in the elements thereby decreasing the reliability and yet increasing the power consumption.
In the synchronous rectification PWM driving, when a transistor in the upper arm and the counterpart transistor in the lower arm are turned on simultaneously, an undesirable flow-through current is generated in both the transistors. In order to prevent this flow-through current, there is a need to prepare an OFF period (dead zone) where a transistor in the upper arm and its counterpart transistor in the lower arm are simultaneously turned off. The dead zone is determined, in general, by not the PWM frequency but the switching characteristics of the transistors. For this purpose, a longer dead zone is set in addition to a higher PWM frequency set for increasing the rotational accuracy of the motor, then the dead zone takes a larger part with respect to the PWM duty, and the advantage of reducing the power loss is thus lowered. On the contrary, assuming that a priority is given to the reduction of power loss, and set the dead zone without any margin, then characteristics-dispersion of the transistors invites a shortage of the dead zone and generates the flow-through current. In order to overcome this problem about the dead zone, expensive switching elements having excellent switching characteristics are needed or a complicated circuit is needed for realizing an accurate dead zone. Therefore the cost increase must be admitted in any way.
In the synchronous rectification PWM driving, a change in the r.p.m. of a motor, a change in a load, or an instruction of reducing torque narrows the PWM duty, so that the regenerative current passing through the driving coils back-flows to the power supply (hereinafter referred to as a negative current). On top of that, when the negative current flows, the power supply voltage increases depending on an impedance of the power supply, and this voltage rise sometimes causes inconvenience to the motor, the motor driving apparatus, and the device incorporating those components.